The present invention relates to rare-earth doped ceramic scintillators for computerized tomography (CT) and other x-ray, gamma radiation, and nuclear radiation detecting applications. More specifically, the invention relates to rare-earth-doped, polycrystalline, yttria/gadolinia (Y.sub.2 O.sub.3 /Gd.sub.2 O.sub.3) ceramic scintillators.
Computerized tomography scanners are medical diagnostic instruments in which the subject is exposed to a relatively planar beam or beams of x-ray radiation, the intensity of which varies in direct relationship to the energy absorption along a plurality of subject body paths. By measuring the x-ray intensity (i.e., the x-ray absorption) along these paths from a plurality of different angles or views, an x-ray absorption coefficient can be computed for various areas in any plane of the body through which the radiation passes. These areas typically comprise approximately a square portion of about 1 mm.times.1 mm. The absorption coefficients are used to produce a display of, for example, the bodily organs intersected by the x-ray beam.
An integral and important part of the scanner is the x-ray detector which receives the x-ray radiation which has been modulated by passage through the particular body under study. Generally, the x-ray detector contains a scintillator material which, when excited by the impinging x-ray radiation, emits optical wavelength radiation. In typical medical or industrial CT applications, the optical output from the scintillator material is made to impinge upon photoelectrically responsive materials in order to produce electrical output signals, the amplitude of which is directly related to the intensity of the impinging x-ray radiation. The electrical signals are digitized for processing by digital computer means which generates the absorption coefficients in a form suitable for display on a cathode ray tube screen or other permanent media.
Due to the specific and demanding computerized tomography requirements, not all scintillator materials which emit optical radiation upon excitation by x-ray or gamma ray radiation are suitable for CT applications. Useful scintillators must be efficient converters of x-ray radiation into optical radiation in those regions of the electromagnetic spectrum (visible and near visible) which are most efficiently detected by photosensors such as photomultipliers or photodiodes. It is also desirable that the scintillator transmit the optical radiation efficiently, avoiding optical trapping, such that optical radiation originating deep in the scintillator body escapes for detection by externally situated photodetectors. This is particularly important in medical diagnostic applications where it is desirable that x-ray dosage be as small as possible to minimize patient exposure, while maintaining adequate quantum detection efficiency and a high signal-to-noise ratio.
Among other desirable scintillator material properties are short afterglow or persistence, low hysteresis, high x-ray stopping power, and spectral linearity. Afterglow is the tendency of the scintillator to continue emitting optical radiation for a time after termination of x-ray excitation, resulting in blurring, with time, of the information-bearing signal. Short afterglow is also highly desirable in applications requiring rapid sequential scanning such as, for example, in imaging moving bodily organs. Hysteresis is the scintillator material property whereby the optical output varies for identical x-ray excitation based on the radiation history of the scintillator. This is undesirable due to the requirement in CT for repeated precise measurements of optical output from each scintillator cell and where the optical output must be substantially identical for identical x-ray radiation exposure impinging on the scintillator body. Typical detecting accuracies are on the order of one part in one thousand for a number of successive measurements taken at relatively high rate. High x-ray stopping power is desirable for efficient x-ray detection. X-rays not absorbed by the scintillator escape detection. Spectral linearity is another important scintillator material property because x-rays impinging thereon have different frequencies. Scintillator response must be substantially uniform at all x-ray frequencies.
Among scintillator phosphors considered for CT use are monocrystalline materials such as cesium iodide (CsI), bismuth germanate (Bi.sub.4 Ge.sub.3 O.sub.12), cadmium tungstate (CdWO.sub.4), and sodium iodide (NaI). Many of the aforementioned materials typically suffer from one or more deficiencies such as excessive afterglow, low light output, cleavage, low mechanical strength, hysteresis, and high cost. Many monocrystalline scintillators are also subject to hygroscopic attack. Known polycrystalline scintillators are efficient and economical. However, due to their polycrystalline nature, such materials are not efficient light propagators and are subject to considerable optical trapping. Internal light paths are extremely long and tortuous, resulting in unacceptable attenuation of optical output.
Fabrication of monocrystalline scintillators from multicomponent powder constituents is typically not economical and is frequently impractical. The multicomponent powder composition must be heated to a temperature above its melting point, and ingots of dimensions larger than those of each detector channel are grown from the melt. Considering the size of the bars required and the temperatures involved, the process is difficult in and of itself. In addition, some materials exhibit phase changes while cooling, which would cause the crystals to crack when cooled after the growing process. Furthermore, single crystals tend to be susceptible to the propogation of lattice defects along the crystal planes.
U.S. Pat. No. 4,242,221 issued to D. A. Cusano et al (assigned to the same assignee as the present invention) describes methods for fabricating polycrystalline phosphors into ceramic-like scintillator bodies for use in CT.
The present invention provides improved ceramic scintillators composed of yttria-gadolinia and including a variety of rare earth activators for enhancing luminescent efficiency.
The terms "transparency" and "translucency", as used herein, describe various degrees of optical clarity in the scintillator material. Typically, the inventive scintillator materials exhibit an optical attenuation coefficient of less than 100 cm.sup.-1, as measured by standard spectral transmittance tests (i.e., "narrow" angle transmission) on a polished scintillator material plate, at the luminescent wavelength of the respective ion. The most desirable scintillator materials have lower attenuation coefficients and hence higher optical clarity (transparency).